Since polyimide and a precursor thereof have excellent durability, heat resistance, flame retardancy, mechanical and electronical properties, they are actively used as a base film of a printed circuit board, and a high integrated semiconductor device or a cover film for high integrated multilayered wire board. In the case of a flexible print wiring board, it is a base film that is used in conjunction with copper (Cu) of a copper Clad Laminate (hereinafter, referred to as ‘CCL’) on which a conductive circuit pattern is formed and a polyimide cover lay film that is used by attaching an adhesive in order to protect a circuit of a conductive circuit pattern and improve the resistance to bendability. The cover lay film is previously punched and processed, and layered on the CCL surface on which the circuit is formed by using the thermal compression method. However, in accordance with the miniaturization and the multifunction of electronic devices, and in particular, the lightness, slimness, shortness, and miniaturization of the portable devices, the design of circuit board used in the electronic devices becomes more highly density, thus the circuit becomes fine. However, a known method in which a hole is formed on a polyimide cover lay film and an operator aligns it with the circuit on the CCL has a problem in that since the position precision is low, the current cover lay is insufficient to cope the high density mounting. In addition, in the case of the method in which by using a CO2 laser or an excimer laser, holes are formed on a cover lay film, the position precision is good, but there is a disadvantage in that equipment cost and equipment maintaining cost are high.
Accordingly, currently, in order to solve this problem, a method in which by using a photosensitive resin composition, the fineness of a circuit pattern and a position precision are improved by a photolithography process has been suggested. In the case of when the photosensitive resin composition is used, fine holes that are required to connect a circuit are capable of being precisely formed at a desired position by thermal compressing a liquid or film type photosensitive resin composition on a circuit of a CCL, UV exposing it according to the pattern, developing it by using a developing solution, washing it with water, drying it, and thermally curing it.
In the case of the developing solution that is used in the developing process, in general, a combustible organic solvent or a toxic solvent is used, and they have a problem in terms of operation stability. Accordingly, currently, a photosensitive resin composition that is advantageous in terms of operation stability and is capable of being developed by a weak alkali aqueous solution is required, and many studies thereof have been made. In order to develop it in respects to the alkali aqueous solution, in the case of when a photosensitive resin composition including a base resin and a functional group such as a carbonic acid, an alcoholic group such as phenol, or a multibase acid is used, there are problems in that the developability in respects to the alkali aqueous solution is increased, but resistance to solvent, resistance to chemicals, and dielectricity are reduced, thus it is not suitable to circuit boards such as FPCB (Flexible Printed Circuit Board). In addition, in the case of the photosensitive resin composition that is produced by adding acrylate and the like to known epoxy resin that is used in the dry film, since the flame retardancy property is poor and the soldering heat resistance after the curing is insufficient, there are problems in that the resin is discolored during the soldering and the delamination occurs in circuits. In addition, since the flexibility and the resistance to bendability are poor, while the repeating folding occurs, cracks are easily formed, thus it is not suitable to be used as a protective film of a circuit board.
In order to solve these problems, a demand for a photosensitive resin for circuit protection on the basis of polyimide that has the high heat resistance, resistance to bendability and dielectric property, and is applied to a known protective film for circuit pattern is high. However, even though these requirements are present, there are several technical problems in use of polyimide as a material of a photosensitive circuit protective film. In the case of when polyimide is used as the photosensitive resin, it is used in a polyamic acid form that is a precursor of polyimide which is advantageous of shaping, and in this case, the high temperature of not less than 350° C. is required to polyimidize the polyamic acid. Therefore, in the case of when it is polyimidized through a thermal curing process while it is coated on the circuit pattern in a polyamic acid form or attached in a film form, there are problems in that the copper circuit that is weak to heat may be oxidized and deteriorated, which is undesirable. In addition, in the photosensitive protective film, voids between fine patterns must be filled, in general, since polyimide has high modulus, it is not easy to realize the filling property, and since resistance to chemicals is excellent, it is hard to realize the developability in respects to the weak alkali aqueous solution, there is a limit in development of a circuit protective film that is made of polyimide.